In a digitally controlled inkjet printing system, a receiver media (also referred to as a print medium) is conveyed past a series of components. The receiver media can be a cut sheet of receiver media or a continuous web of receiver media. A web or cut sheet transport system physically moves the receiver media through the printing system. As the receiver media moves through the printing system, liquid (e.g., ink) is applied to the receiver media by one or more printheads through a process commonly referred to as jetting of the liquid. The jetting of liquid onto the receiver media introduces significant moisture content to the receiver media, particularly when the system is used to print multiple colors on a receiver media. Due to the added moisture content, an absorbent receiver media expands and contracts in a non-isotropic manner, often with significant hysteresis. The continual change of dimensional characteristics of the receiver media can adversely affect image quality. Although drying is used to remove moisture from the receiver media, drying can also cause changes in the dimensional characteristics of the receiver media that can also adversely affect image quality.
FIG. 1 illustrates a type of distortion of a receiver media 3 that can occur during an inkjet printing process. As the receiver media 3 absorbs the water-based inks applied to it, the receiver media 3 tends to expand. The receiver media 3 is advanced through the system in an in-track direction 4. The perpendicular direction, within the plane of the un-deformed receiver media, is commonly referred to as the cross-track direction 7. Typically, as the receiver media 3 expands (or contracts) in the cross-track direction 7, contact between the receiver media 3 and contact surface 8 of rollers 2 (or other web guiding components) in the inkjet printing system can produce sufficient friction such that the receiver media 3 is not free to slide in the cross-track direction 7. This can result in localized buckling of the receiver media 3 away from the rollers 2 to create lengthwise flutes 5 (also called ripples or wrinkles) in the receiver media 3. Wrinkling of the receiver media 3 during the printing process can lead to permanent creases in the receiver media 3 which adversely affects image quality.
Commonly-assigned U.S. Pat. No. 8,303,106 to Kasiske et al., entitled “Printing system including web media moving apparatus” and U.S. Pat. No. 8,303,107 to Kasiske et al., entitled “Printing method including web media moving apparatus,” both of which are incorporated herein by reference, disclose a) a printing system having a printhead that moistens at least a portion of a web of print media, and b) a roller including a pattern of recesses and ridges, so that the web contacts a portion of the roller downstream of the printhead. The recesses and ridges help compensate for cross track expansion of the print media caused by absorption of water-based ink and also help reduce the likelihood of wrinkling of the print media. Also disclosed as shown in FIG. 2 (a copy of FIG. 9 in the aforementioned U.S. Pat. No. 8,303,106) is a drive roller 100 having an alternating pattern of ridges 106 and recesses that are positioned along the axis of rotation 102. Drive roller 100 is divided into a first section 108, a second section 110, and a third section 112, where the second section 110 is located between the outer first and third sections 108, 112. Drive roller 100 includes a concave profile, such that the diameter of the ridges 106 located in the first section 108 and the third section 112 of the drive roller 100 are greater than the diameter of the ridges 106 located in the central second section 110 of the roller 100. Drive roller 100 is driven, for example, by motor 152. A first nip roller 154 is positioned to engage a first ridge 156 of the ridges 106 of drive roller 100 and a second nip roller 158 is positioned to engage a second ridge 160 of the ridges 106 of drive roller 100. The first ridge 156 is located proximate to a first edge 162 of print media, and the second ridge 160 is located proximate to a second edge 164 of the receiver media 3 (FIG. 1).
U.S. Patent Application Publication 2010/0054826 to Hieda, entitled “Web transfer method and apparatus,” discloses a web control system that includes a tiered roller and a pair of nip rollers. The tiered roller is formed to have a larger diameter at both ends than in a central portion. The nip rollers are arranged to incline outward to spread the web as it passes between the tiered roller and the nip rollers.
Recently it has been found that wrinkling can occur for lighter weight papers (densities on the order of 100 grams per square meter or less) if the nip rollers for a drive roller are located near the edges of the web. In addition, during start up of the printing system, the web can shift back and forth along the cross-track direction. If the nip rollers are located near the edges of the web of print media, they can move off the edges and cause web breaks. Furthermore, drive rollers having the nip rollers positioned at the ends of the roller as in FIG. 2 cannot accommodate a wide range of web widths. For narrower media, one or both nip rollers will not be in contact with the web.
What is needed is a drive roller configuration having a profile that compensates for dimensional changes in the web to reduce wrinkling in the web, while also providing reliable engagement of the nip rollers with a wide range of receiver media widths, even during start up of the system